Generally, an energy storage system, an electric vehicle and the like require a chargeable electricity storage system, in which the electricity storage system includes a plurality of batteries. Each battery has a deviation in capacitance which occurs due to several reasons such as a process for manufacturing a battery.
The battery may be configured and used in a larger form of batteries, such as battery unit cells in a minimum unit, battery modules manufactured in one module form in which the plurality of battery unit cells are connected to each other, battery trays manufactured in a tray form in which the plurality of battery modules are connected to each other, and battery banks manufactured in a larger form by connecting the plurality of battery trays to each other, as needed and may be assigned with various names, but these batteries have the same basic function to store electricity. Further, these batteries may be connected in series, in parallel, or in a serial and parallel mixing structure.
Generally, the electricity storage system in which the batteries are connected in series, in parallel, or in a serial and parallel mixing structure has a voltage unbalance due to different electrochemical characteristics between the respective batteries configuring the electricity storage system.
Therefore, a deviation occurs in charging and discharging voltages of each battery during a charging and discharging cycle of the batteries. Therefore, a specific battery may be overcharged while the battery is charged and a specific battery may also be over discharged while the battery is discharged. As described above, the overcharging or the over discharging of the specific battery among the batteries is a cause which may reduce performance of the battery, deteriorate the battery, and reduce the lifespan of the battery.
Therefore, the battery balancing to control each of the voltage differences between the plurality of cells in the battery to be in a tolerance or the voltages of the plurality of cells to be equal is important and therefore research into a balancing circuit using a voltage and a state of charge (SOC) to remove the voltage unbalance has been prevalently conducted.
US Patent Application Publication No. US-2011-0025258 discloses a system for scheduling charging and discharging of a battery.